The fact that ions exist in a spark plug gap after fuel combustion can be utilized for many purposes. For example, there is a known relationship between ionization current and engine misfires.
An ion detection system in its simplest form consists of a DC power supply (usually +100 volts or greater), a series resistor in the multi-megohm range, and the center electrode of the spark plug in the cylinder (the ion probe). The technique of ion detection is well known and dates back to the 1930's.
Ion detection is a difficult technique to apply because of electromagnetic interference present in a vehicle. In particular, the presence of noise (spark event) three orders of magnitude larger than the signal and on the signal line creates severe problems. Difficulties also occur when sensing a microampere level signal in the presence of the relatively high DC voltage supply used to detect ions. As illustrated in U.S. Pat. Nos. 5,180,984; 5,207,200; 5,087,882; and 4,862,093, previous investigators have used capacitive coupling to remove the DC voltage, but this can produce difficulties with recovery time from the large transients (spark events) if long time constant coupling (large capacitor) is used. In either case, a reduction in signal-to-noise ratio is experienced.
Other investigators have used an isolated power supply (neither output terminal grounded), with an ion current sensing resistor between the return line and circuit ground. This eliminates the distortion problems referred to previously, but is a very inconvenient configuration and tends to be noisy.